The problem with adenovirus vaccines is that body might build immunity against the adenovirus as well. So, you give one jab, adenovirus infects enough cells and creates enough antigens, immune systems notices and does something about it, but it also notices the adenovirus and makes antibodies against that, too.

With the second jab those antibodies immediately neutralize the adenovirus so it can't infect enough cells and make enough of the COVID antigens so the protection is not that strong.

The Russian vaccine avoids it by using two different strains of adenovirus for the two jabs